1. Field of the Invention
The present invention concerns insulating ceramic substances having controlled porosity and the method for preparing them by sintering.
2. Description of the Prior Art
It is known that numerous modern techniques relating to the treatment of molten metals or to the circulating thereof require, on the part of the materials placed in contact with the liquid metals, properties which are fairly difficult to conciliate:
The material must be passive with respect to the molten metal which may be very corrosive when hot, as is the case with aluminium or steel; PA1 The material must bear, without cracking, repeated thermal shocks which are very great, resulting from the sudden change of the ambient temperature to the temperature of the molten metal and vice versa; PA1 The material must, moreover, have, when hot, well-defined properties with respect to electricity : remain insulant or conductive, according to the use which is made thereof; lastly, the material must remain stable in an oxydising atmosphere. PA1 Mixing of fine powders of oxydes of elements chosen from groups III and IV of the periodic classification of elements; PA1 Sintering under high pressure, at a high temperature, of a part of the mixture of fine powders of oxydes of elements chosen from groups III and IV with a view to obtaining a very dense product; PA1 Crushing of the sintered product thus obtained; PA1 Mixing of the crushed product with the remainder of the fine powder obtained during the first mixing operation; PA1 Mulling of the mixture of the crushed product with the fine powder after addition of 0.1 % to 6 % of the total weight of a fluoride of an element of the group IA of the periodic classification of elements; PA1 Proceeding with the vibrating in a mould of the preceding mulled product during a short period;
It is known that, since a very long time, it is a known method to mix fine powders of oxydes in group IV, sinter them under pressure at a high temperature with a view to obtaining a very dense product, crush these grains and sinter under charge the preceding product. A very compact product, cracking rapidly if it is subjected to these temperature cycles, is thus obtained.
It is also known that the fine powder mulled with an addition of a fluoride of an element in group IA has also been used and has also led to the manufacturing of a compact ceramic product. Indeed, the fluorine ion which is probably released tends to make the adhering of the grains together easier.
It will be seen therefore that these various known methods make it possible to obtain a compact ceramic substance whose properties cannot satisfy the requirements mentioned above.